Drag and drop may leave keyboard focus unset
When starting a DnD operation, mutter would remove keyboard focus from the client, only to restore it on the data offer destroy.
However, if the DnD fail, the keyboard focus is not restored, leaving the user unable to type in the focused window, even after clicking in the window.
That issue would show only on first attempt, as further DnD attempts would destroy the previous data offer which would also restore the keyboard focus.
Make sure we restore the keyboard focus on drag end as well.